Ah, 120Hz. Like many features used by TV manufacturers to induce people to spend more money on step-up models (see also 1080p, HDMI 3.0), its visible impact on picture quality is often difficult to discern. In test patterns, 120Hz can reduce blurring, and when paired with a 1080p/24 source it can make film-based sources appear a bit smoother, but for the average viewer, and in many cases even experienced reviewers like me, the differences appear slight.
It's easy to confuse 120Hz with dejudder processing, which can have a significant visible impact on picture quality--not all of it good, but that's another story. The confusion increases because many manufacturers market 120Hz and dejudder in the same breath, and, in fact, nearly every 120Hz LCD also features dejudder. The Sharp LC-46D85U we just reviewed is one exception.